Spatial and seasonal variability of nitrous oxide in a large freshwater lake in the lower reaches of the Yangtze River, China

Denitrification regulates spatiotemporal pattern of N2O emission in an interconnected urban river-lake network

Urban rivers are hotspots of N2O production and emission. Interconnected river-lake networks are constructed to improve the water quality and hydrodynamic conditions of urban rivers in many cities of China. However, the impact of the river-lake connectivity project on N2O production and emission remains unclear. This study investigated dissolved N2O and emission of the river-lake network in Wuhan City, China from March 2021 to December 2021. The results showed that river-lake connection greatly decreased riverine Nitrogen (N) concentration and increased dissolved oxygen (DO) concentration compare to traditional urban rivers. N2O emissions from the urban river interconnected with lakes (LUR: 67.3 ± 92.6 μmol/m2/d) were much lower than those from the traditional urban rivers (UR: 467.3 ± 1075.7 μmol/m2/d) and agricultural rivers (AR: 20.4 ± 15.3μmol/m2/d). Regression tree analysis suggested that the N2O concentrations were extremely high when hypoxia exists (DO < 1.6 mg/L), and TDN was the primary factor regulating N2O concentrations when hypoxia does not occur. Thus, we ascribe the low N2O emission in the LUR and AR to the lower N contents and higher DO concentrations. The microbial process of N2O production and consumption were quantitatively estimated by isotopic models. The mean proportion of denitrification derived N2O (fbD) was 63.5 %, 55.6 %, 42.3 % and 42.7 % in the UR, LUR, lakes and AR, suggested denitrification dominated N2O production in the urban rivers, but nitrification dominated N2O production in the lakes and AR. The positive correlation between logN2O and fbD suggested that denitrification is the key process to regulate the N2O production and emission. The abundance of denitrification genes (nirS and nirK) was much higher than that of nitrification genes (amoA and amoB), also evidenced that denitrification was the main N2O source. Therefore, river-lake interconnected projects changed the nutrients level and hypoxic condition, leading to the inhibition of denitrification and nitrification, and ultimately resulting in a decrease of N2O production and emission. These results advance the knowledge on the microbial processes that regulate N2O emissions in inland waters and illustrate the integrated management of water quality and N2O emission.

Methane ebullition fluxes and temperature sensitivity in a shallow lake

Inland waters are important sources of atmospheric methane (CH4), with a major contribution from the CH4 ebullition pathway. However, there is still a lack of CH4 ebullition flux (eFCH4) and their temperature sensitivity (Q10) in shallow lakes, which might lead to large uncertainties in CH4 emission response from aquatic to climate and environmental change. Herein, the magnitude and regulatory of two CH4 pathways (ebullition and diffusion) were studied in subtropical Lake Chaohu, China, using the real-time portable greenhouse gas (GHG) analyzer-floating chamber method at 18 sites over four seasons. eFCH4 (12.06 ± 4.10 nmol m-2 s-1) was the dominant contributing pathway (73.0 %) to the two CH4 emission pathways in Lake Chaohu. The whole-lake mass balance calculation demonstrated that 56.6 % of the CH4 emitted from the sediment escaped through the ebullition pathway. eFCH4 was significantly higher in the western (WL: 16.54 ± 22.22 nmol m-2 s-1) and eastern lake zones (EL: 11.89 ± 15.43 nmol m-2 s-1) than in the middle lake zone (ML: 8.86 ± 13.78 nmol m-2 s-1; p < 0.05) and were significantly higher in the nearshore lake zone (NL: 15.94 ± 19.58 nmol m-2 s-1) than in the pelagic lake zone (PL: 6.64 ± 12.37 nmol m-2 s-1; p < 0.05). eFCH4 was significantly higher in summer (32.12 ± 13.82 nmol m-2 s-1) than in other seasons (p < 0.05). eFCH4 had a strong temperature dependence. Sediment total organic carbon (STOC) is an important ecosystem level Q10 driver of eFCH4. The meta-analysis also verified that across ecosystems the ecosystem-level Q10 of eFCH4 was significantly positively correlated with STOC and latitude (p < 0.05). This study suggests that eFCH4 will become increasingly crucial in shallow lake ecosystems as climate change and human activities increase. The potential increase in ebullition fluxes in high-latitude lakes is of great importance.

NosZ-II-type N2O-reducing bacteria play dominant roles in determining the release potential of N2O from sediments in the Pearl River Estuary, China

The microbial reduction of N₂O serves as a "gatekeeper" for N₂O emissions, determining the flux of N₂O release into the atmosphere. Estuaries are active regions for N₂O emissions, but the microbial functions of N₂O-reducing bacteria in estuarine ecosystems are not well understood. In this study, the ¹⁵N isotope tracer method, qPCR, and high-throughput sequencing were used to analyze N₂O production, reduction, and emission processes in surface sediments of the Pearl River Estuary. The ¹⁵N isotope tracer experiment showed that the N₂O production rates declined and the N₂O reduction potential (R_r, the ratio of N₂O reduction rates to N₂O production rates) increased from upstream to downstream of the Pearl River Estuary, leading to a corresponding decrease of the N₂O emission rates from upstream to downstream. The gene abundance ratio of nosZ/nir gradually increased from upstream to downstream and was negatively correlated with the water N₂O saturation. The gene abundance of nosZ II was significantly higher than that of nosZ I in the estuary, and the nosZ II/nosZ I abundance ratio was positively correlated with N₂O reduction potential. Furthermore, the community composition of NosZ-I- and NosZ-II-type N₂O-reducing bacteria shifted from upstream to downstream. NosZ-II-type N₂O-reducing bacteria, especially Myxococcales, Thiotrichales, and Gemmatimonadetes species, contributed to the high N₂O reduction potential in the downstream. Our results suggest that NosZ-II-type N₂O-reducing bacteria play a dominant role in determining the release potential of N₂O from sediments in the Pearl River Estuary. This study provides a new insight into the function of microbial N₂O reduction in estuarine ecosystems.

Large alpine deep lake as a source of greenhouse gases: A case study on Lake Fuxian in Southwestern China

Freshwater lakes are recognized as potential sources of greenhouse gases (GHGs) that contribute to global warming. However, the spatiotemporal patterns of GHG emissions have not been adequately quantified in large deep lakes, resulting in substantial uncertainties in the estimated GHG budgets in global lakes. In this study, the spatial and seasonal variability of diffusive GHG (CO₂, CH₄, and N₂O) emissions from Lake Fuxian located on a plateau in Southwestern China were quantified. The results showed that the surface lake water was oversaturated with dissolved GHG concentrations, and the average concentrations were 24.25 μM CO₂, 0.044 μM CH₄, and 14.28 nM N₂O, with diffusive emission rates of 8.82 mmol CO₂ m^-2 d^-1, 31.94 μmol CH₄ m^-2 d^-1, and 4.94 μmol N₂O m^-2 d^-1, respectively. Diffusive CH₄ flux exhibited high temporal and spatial variability similar to that in most lakes. In contrast, diffusive CO₂ and N₂O flux showed distinct seasonal variability and similar spatial patterns, emphasizing the necessity for increasing the temporal resolution in GHG flux measurements for integrated assessments. Water temperature and/or oxygen concentrations were crucial in regulating seasonal variability in GHG emissions. However, no limnological parameter was found to govern the spatial GHG patterns. The frequent advection mixing caused by wind-driven currents might be the reason for the low spatial heterogeneity in GHGs, in which the inconspicuous mechanism requires further research. It was recommended that at least 11 locations were needed for representative whole lake flux estimates at each sampling campaign. In addition, the maximum peak of CH₄ in the oxycline from Lake Fuxian indicated that low CH₄ oxidation occurred in oxic waters. Overall, this study suggests that, compared to other tropical and temperate lakes, this alpine deep lake is a minor CO₂ and CH₄ source, but a moderate N₂O source, which are horizontally uniform.

Submerged macrophytes regulate diurnal nitrous oxide emissions from a shallow eutrophic lake: A case study of Lake Wuliangsuhai in the temperate arid region of China

Submerged macrophytes can increase oxygen concentrations of water and promote diel oxygen fluctuations, and this phenomenon is hypothesized to play a vital role in regulating nitrous oxide (N₂O) emissions from eutrophic lakes. However, the effects of submerged macrophytes on N₂O emissions in shallow eutrophic lakes remain poorly investigated. In this study, Lake Wuliangsuhai, a typical shallow eutrophic lake, was investigated to study the role of submerged macrophytes in regulating N₂O emissions. We measured the N₂O fluxes and related parameters through continual 72-h in situ diel monitoring in two sampling sections that covered dense submerged macrophyte areas and open water. In this study, the dissolved oxygen (DO) concentration of the water in the submerged macrophyte area exhibited significant diurnal variations, with significantly higher water oxygen concentrations than the open water area during the daytime. The N₂O fluxes of Lake Wuliangsuhai ranged from 0.01 to 0.24 μmol m^-2 h^-1, with an average value of 0.11 μmol m^-2 h^-1. Moreover, significant diel variations in the N₂O flux and net N₂O production were observed in the submerged macrophyte areas, where the maximum N₂O flux occurred at midday. The molar ratios of NH₄⁺-N to oxygen (N/O ratio) of the water were responsible for the diel variations in the N₂O production in the lake. However, the high oxygen concentration of the water was the major regulator of the N₂O flux of Lake Wuliangsuhai. Therefore, submerged macrophyte restoration is significant not only for water quality improvement in shallow eutrophic lakes but also for N₂O emission mitigation by increasing the DO concentration of the water.

Nonlinear pattern and algal dual-impact in N2O emission with increasing trophic levels in shallow lakes

Shallow lakes are considered important contributors to emissions of nitrous oxide (N₂O), a powerful greenhouse gas, in aquatic ecosystems. There is a large degree of uncertainty regarding the relationship between N₂O emissions and the progress of lake eutrophication, and the mechanisms underlying N₂O emissions are poorly understood. Here, N₂O emission fluxes and environmental variables in different lakes along a trophic state gradient in the Yangtze River basin were studied. N₂O emission fluxes were -1.0-53.0 μg m^-2 h^-1 and 0.4-102.9 μg m^-2 h^-1 in summer and winter, respectively, indicating that there was marked variation in N₂O emissions among lakes of different trophic state. The non-linear exponential model explained differences in N₂O emission fluxes by the degree of eutrophication (p < 0.01). TN and chl-a both predicted 86% of the N₂O emission fluxes in shallow lakes. The predicted N₂O emission fluxes based on the IPCC EF_5r overestimated the observed fluxes, particularly those in hyper-eutrophic lakes. These findings demonstrated that nutrient-rich conditions and algal accumulation are key factors determining N₂O emission fluxes in shallow lakes. Furthermore, this study also revealed that temperature and algae accumulation-decomposition determine an N₂O emission flux in an intricate manner. A low temperature, i.e., winter, limits algae growth and low oxygen consumption for algae decomposition. The environment leaves a high dissolved oxygen concentration, slowing down N₂O consumption as the final step of denitrification. In summer, with the oxygen consumed by excess algal decomposition, the N₂O production is limited by the complete denitrification as well as the limited substrate supply of nitrate by nitrification in hypoxic or anoxic conditions. Such cascading events explained the higher N₂O emission fluxes from shallow lakes in winter compared with summer. This trend was amplified in hyper-eutrophic shallow lakes after algal disappearance. Collectively, algal accumulation played a dual role in stimulating and impeding N₂O emissions, especially in hyper-eutrophic lakes. This study expands our knowledge of N₂O emissions from shallow lakes in which eutrophication is underway.